1988 gmc sierra 4whl drive doesn't engage.You can hear and feel the gears grind but no power to front wheels

SOURCE: 1990 GMC Sierra 4x4 1500

the connections near the front axle that activate the motor to engage the system aren't melted? That's a comon problem for that body style. If there good, sounds like it could be that motor itself. To figure out if it's you motor or transfercase, jack up the rear end, have a friend inside shift into 4 wheel drive, or try to, and put it in gear and apply some gas. If the front driveshaft is turning then it's something with the front end. If not, farther back in the system.

SOURCE: Four wheel drive will not

sometimes it helps to back the truck up a little ways, for some reason it helps lock it in.

SOURCE: Front drive, 4 Wheel drive won't engage

Check the switch on the transfercase remove the vacume lines if there is oil in them the switch has to be replaced and the lines should be cleaned out you should also check under the battery tray to make sure the boot is not ripped that controls the leaver to engage the front axel

SOURCE: 4 wheel drive will not engage on my 1989 gmc

actually that first soultion is wrong it is correct on smaller pickups but on the full size pick ups they use a differnet sytem
there a electric actuator on front axle
there be two elctrical plugs on front axle u want the bigger of the two other one is just to turn the 4 wheel drive light on
un plug actuator then remove actuator after it removed plug it back in but leave it out of axle turn key on and shift down to 4 wheel drive the actuator should move out if it dont that your problem if i does move this isnt your problem it either the tranfer case or in front axle

SOURCE: 2004 stx f150 right front wheel grinding noise,

I agree with duane_wong on this one ( not like he was looking for my approval :) ), it sounds like a problem with the IWE ( Integrated Wheel End ) on that side.
Quick test, when you are in 4H ( or 4L ) does the noise still happen ?
I would say cracked vacuum line to the IWE, or a problem with the vacuum diaphragm in the IWE. You can test by having the motor running in park, and in 2H, and check the vacuum line at the IWE for vacuum. Make sure you test to the vacuum line, not the vent line ( ends in the engine compartment, not hooked to anything )

This is a block diagram of the IWE vacuum system

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